Conceptual Models > Kinetics and rate controlling factors

Kinetic of methane formation

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MMMM:
Hello every one
I am trying to model methane formation reaction during hydrogen storage. The reaction is
CO2 + 4H2 = CH4 + H2O
I checked the previous related posts and found that the kinetic of this reaction is written as follows.
Hdg -1  H2 +1
Why we dont use
CO2 -1  Hdg -4   CH4 +1  H2O +2  ?

 I appreciate if you can help me.

dlparkhurst:
I don't advise it, but in fact, you could use the formula you suggest. Only the net reaction is added from the formula. In this case, forthe reaction is


--- Code: ---CO2 -1  Hdg -4   CH4 +1  H2O +2

--- End code ---

1 carbon is removed and 1 carbon is added, net 0.
2 oxygen are removed and 2 oxygen are added, net 0.
8 hydrogen are added, net 8.
4 Hdg are removed, net 4.

So the reaction is equivalent to Hdg -4 H2 +4.

So, in general, you should consider the net change in elements due to the reaction. In this case, the reaction is converting the inert version of H2 (Hdg) to the reactive version H2. PHREEQC will react to equilibrium H2 and CO2 to equilibrium, and thermodynamically, H2 will consume CO2, so the rate of CO2 consumption is equal to the rate that H2 is supplied.

MMMM:
Thanks alot

MMMM:
Hello
When we use the above reaction, which only defines Hdg and H2, is methane produced in proportion to the hydrogen consumption? Does the expression gas(CH4(g)) correctly display the amount of methane produced?

dlparkhurst:
I'll say it this way, PHREEQC will calculate a balanced chemical reaction when you add H2. However, it will calculate thermodynamic equilibrium, so, it will react H2 with the thermodynamically favored electron acceptor. If O2 or SO4 are present, the H2 will react with them before generating CH4.

The amount of methane created, in moles, will be the sum of the aqueous and gaseous methane: TOTMOL("C(-4)") + GAS("CH4(g)").

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